It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that when the Messenger of Allah (peace and blessings be upon him) bowed (in ruku'), he kept (his back) so level that if water had been poured on his back it would have stayed in place.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12726